Deflection yokes for television picture tubes comprise pairs of conductor coils on opposite sides of the tube, which are energized with a current for producing a magnetic field having field lines intersecting the electron beam path, the field lines being disposed perpendicular to the beam path. It is known to employ main and auxiliary deflection yokes in a television apparatus. A main yoke section provides a large amplitude electromagnetic deflection of the beam for scanning in the horizontal and vertical directions as needed to define a raster.
An auxiliary yoke section can accomplish a number of additional functions, including, for example, improving the convergence of the individual red, blue and green electron beams in a color television project apparatus. An auxiliary deflection yoke can be provided for defining alphanumeric characters at a position in the raster determined by the main deflection yoke, using a vector scanning of the beam at the desired position. Another possibility is an auxiliary yoke for modulating the beam scan as a function of the video so as to control contrast, which is modulated by the beam scan velocity.
An auxiliary yoke provides a deflection of a smaller amplitude than the main deflection yoke, and can provide deflection at high speed. The auxiliary deflection yoke is placed to the rear of the primary deflection yoke, between the electron guns and the primary deflection yoke.
The magnetic field produced by a coil naturally has a magnetic field intensity that extends spatially from the area of the conductors defining the coil. The field decreases in amplitude with distance from the coil conductors, i.e., with distance along the Z axis. To minimize coupling between an auxiliary deflection coil and a primary deflection coil on the same axis, it is possible to space the auxiliary and primary coils from one another along the Z axis. However, the length of the picture tube is thereby increased. In addition to the physical length of the deflection coils along the Z axis, the operative length of the deflection system as a whole (primary plus auxiliary) determines the focal length of the gun-deflection system, which forms an electron lens. Accordingly, a longer deflection system must be spaced farther from the screen and results in poorer resolution at the screen. A compact deflection yoke arrangement is desirable as it enables a shorter overall tube length.
In a saddle shaped deflection yoke, as shown in Prior Art FIGS. 2 and 10 of the drawings, the ends of the yoke at the axial extremes along the tube are formed such that the windings are superimposed to protrude radially of the tube. In this manner, the magnetic field proceeding axially along the tube is more sharply cut off at the axial end than occurs if the windings at the axial end are superimposed axially along the tube. As shown in Prior Art FIG. 4, the magnetic field intensity proceeding axially tail off to near zero when passing the axial end of a saddle shaped coil of this type. A saddle shaped coil of the type shown in FIGS. 2 and 10 has heretofore been preferred.
It is also possible to provide a high permeability magnetic shunt between the auxiliary coils and the primary coils, e.g., a ferrous ring having a minimum extension along the Z axis as shown in FIG. 10. The magnetic field lines are confined to the high permeability shunt path, tending to localize the fields produced by the respective coils and to better isolate the effects of the primary deflection coils and the auxiliary deflection coils. Notwithstanding these efforts, some coupling of the primary and auxiliary deflection coils remains, in part through the high permeability shunt. Accordingly, modulation of the auxiliary deflection coils by the primary deflection signals (and vice-versa) causes auxiliary deflection to vary with the position of the beam in the raster, and adversely affect convergence and color purity.